The present invention relates to a pulsating combustion system wherein a plurality of pulsating combustors are coupled to each other in parallel.
It is known that a pulsating combustor has many advantages over a conventional burner, e.g., that the pulsating combustor can increase a combustion chamber load to about ten times larger than that of the conventional burner, can achieve high thermal efficiency, does not require a blower for supplying air, and can reduce a toxic component in the exhaust gas. Generally, a pulsating combustor is constituted by a combustion chamber, fuel and air supply lines for respectively supplying fuel and air into the combustion chamber, a tail pipe connected to an exhaust port arranged in the combustion chamber, a movable valve inserted in the fuel supply line, and an ignition system for igniting the gas mixture supplied into the combustion chamber. When the gas mixture in the combustion chamber is ignited, it is explosively burned. The pressure in the combustion chamber is increased so that the movable valve is automatically closed while the burnt gas is exhausted from the exhaust port at a high speed. Upon exhaustion of the gas, the pressure in the combustion chamber becomes a negative pressure (below the atmospheric pressure). The movable valve is then opened to cause the gas mixture to flow into the combustion chamber again. When a predetermined amount of gas flows into the combustion chamber, it is ignited by an after fire to be explosively burned again. The combustion cycle described above is repeated. The combustion in the pulsating combustor is an intermittent explosive combustion. For this reason, noise caused by the pulsating combustor is considerably large.
In order to eliminate this drawback, there is proposed a pulsating combustor, wherein a plurality of, e.g., two pulsating combustors are coupled to each other in parallel. In this pulsating combustion system, noise is reduced by configuring the phases of the strokes constituted by intake, explosive combustion, and exhaust in one of the pulsating combustors as shifted by 180.degree. with respect to those in the other pulsating combustor. This enables and pressure variations between these phases to cancel each other. However, it is difficult to accurately shift the phases by 180.degree. when mechanical movable valves are used to restrict a fluid flow and transmission of pressure variations to the downstream side because strong interference is not caused. Since the movable valves are reciprocated several tens of times per second, a problem of durability is also posed. When the mechanical movable valves are used, a CO--CO.sub.2 characteristic is degraded if a combustion amount range must be widened. Therefore, a turn down ratio (a ratio of a minimum combustion amount to a rated combustion amount) is as low as 2:1 to 3:1 even in the maximum combustion amount range.
Thus, it is proposed in Proceedings of National Heat Transfer Symposium of Japan, p. 725; Ken Kishimoto; May 27, 1986 that in place of the mechanical valves, nozzle-like aerodynamic valves be used, wherein a forward flow coefficient is higher than a reverse flow coefficient. If the aerodynamic valves are used, oscillation cycles of the two pulsating combustors can be shifted by 180.degree. because the pressures in the two combustion chambers strongly interfere with each other through the aerodynamic valves. Since no mechanically moved element is required, the problem of durability can be eliminated. The combustion amount range can be widened to such an extent that a turn down ratio of 10:1 can be obtained without degrading the CO--CO.sub.2 characteristic.
However, even in the pulsating combustion system incorporating the aerodynamic valves, the following problems are present. In order to accurately shift the phases by 180.degree. as described above, a gas mixture taken into the combustion chamber must be ignited at a predetermined timing. In order to prevent a delay in ignition timing, ignitability of the gas mixture must be improved by properly mixing fuel with air. However, in the conventional pulsating combustion system incorporating the aerodynamic valves, fuel and air tend to be insufficiently mixed because of the presence of the aerodynamic valves, resulting in a delay in ignition timing, an increase in noise, and degradation of combustibility. Since the combustion chamber communicates with an air intake system through the aerodynamic valve, when explosive combustion takes place in the combustion chamber, fuel flows into the air intake system by the pressure of explosive combustion. Hence, combustion may take place in the air intake system.